1. Technical Field
Embodiments of this disclosure generally relate to an electrophotographic image forming apparatus, such as a copier and a printer, and to an image density control method employed by the image forming apparatus.
2. Related Art
Electrophotographic image forming apparatuses usually adjust the density of the images they form at predetermined times to keep image quality stable despite environmental changes over time.
For example, density adjustment control may start with forming a patch pattern on a photoconductor image carrier. The patch pattern is composed of a plurality of patch toner images formed at different electric potentials, that is, different imaging conditions. Then, an amount of toner is detected for each patch toner image of the patch pattern is detected. A linear approximation formula is obtained of a developing potential and an amount of toner according to a detected amount of toner to obtain a developing gamma and a developing start voltage according to a gradient of the linear approximation formula. Target charging potential, exposure potential, and developing bias are determined according to the developing gamma and the developing start voltage thus obtained to adjust a developing bias according to the target developing bias. Accordingly, the image density is kept stable from a highlighted portion to a solid portion.
Some electrophotographic image forming apparatuses employ a contact DC charging method to charge the photoconductor image carriers with a charging roller. The contact DC charging method has an advantage over other charging methods, such as using a scorotron charger and using a contact or non-contact AC charging roller, because of its simple configuration and low production cost. On the other hand, the DC charging method is at a disadvantage compared to the other methods when it comes to evenly charging the latent image carriers because of several factors, such as resistance fluctuation due to roller contamination and fluctuation in the width of an electric discharging area, that is, a contact area between the charging roller and the latent image carrier. Even in the early period of image formation, a rough surface of the charging roller may generate an unevenly charged surface of the latent image carrier, and therefore, the potential is unevenly generated on the surface of the latent image carrier. As a result, a problem called background contamination arises such that the toner may adhere to a background (i.e., unexposed portion) of the latent image carrier.
A larger background potential is effective to prevent such background contamination. The background potential is a difference between a charging potential at the background of the latent image carrier charged and a developing bias applied to develop a latent image (i.e., exposed portion) formed on the latent image carrier with toner. A relatively large background potential may not cause any crucial problem when using a single-component developer without carrier. By contrast, when using a two-component developer including carrier, a relatively large background potential may cause a problem called carrier adhesion such that the carrier particles adhere to the background of the latent image carrier. In short, when using the two-component developer, a relatively small background potential may cause background contamination while a relatively large background potential may cause carrier adhesion.
One approach to preventing carrier adhesion involves using carrier having a relatively large particle diameter and/or high resistance. However, such carrier may cause insufficient development of the latent image formed on the latent image carrier, thereby degrading image quality.
Accordingly, a combination of the contact DC charging method and the two-component developer is typically employed by monochrome image forming apparatuses rather than color image forming apparatuses that prioritize image quality.
However, recent demand for inexpensive color image forming apparatuses capable of forming higher-quality images increases demand for practical use of color image forming apparatuses employing the combination of the contact DC charging method and the two-component developer including low-resistant carrier having a relatively small particle diameter to prevent both background contamination and carrier adhesion.